Testing EMDA gravity with observed shadows of M87* and Sgr A*

Abstract

Einstein-Maxwell-dilaton-axion (EMDA) gravity provides a simple framework to investigate the signatures of string theory. The axion and the dilaton fields arising in EMDA gravity have important implications in inflationary cosmology and in addressing the late time acceleration of the Universe. It is therefore instructive to explore the implications of such a model in explaining the astrophysical observations. The Kerr-Sen metric represents the exact, stationary, and axisymmetric black hole solution of EMDA gravity. Such a black hole is characterized by the angular momentum 𝑎 acquired from the axionic field and the dilatonic charge 𝑟2 arising from string compactifications. We study the role of spin and the dilaton parameter in modifying the shape and size of the black hole critical curve, which is associated with the projection of the spherical null geodesics on the sky. We compare the theoretically derived critical curve with the Event Horizion Telescope results related to the images of M87* and Sgr A* to obtain constraints on the dilaton parameter 𝑟2. We take into account the errors in mass and distance of M87* and Sgr A* while deriving their theoretical critical curve. Our analysis reveals that the image of M87* exhibits a preference toward the Kerr scenario when the critical curve angular diameter is calculated with the central value of mass and distance. When errors in mass and distance are taken into account the allowed range of 𝑟2 turns out to be 0≲𝑟2≲1. For Sgr A*, the preferred range of 𝑟2 is 0.1≲𝑟2≲0.4 when central values of mass and distance are used to calculate the theoretical critical curve. When error bars in mass and distance are used to calculate the theoretical critical curve of Sgr A*, the preferred range of 𝑟2 turns out to be 0≲𝑟2≲0.5. Thus the image of M87* favors the Kerr scenario and allows the Kerr-Sen scenario only when errors in the mass and distance are taken into consideration while the image of Sgr A* favors the Kerr-Sen scenario and allows general relativity when errors in the mass and distance are taken into account.